With the advent of portable computing and hand held communication devices there is a need for clean and portable energy sources. The increased functionality and “on time” of these devices represents a challenge for traditional battery technology. Current rechargeable battery systems have significant limitations in the areas of specific energy (watt-hours/kilogram) and energy density (watt-hours/liter).
Fuel cells offer an attractive alternative to rechargeable batteries for portable applications, offering significant performance advantages over current Li-ion cells. One of the most promising fuel cell technologies is a proton exchange membrane (PEM) fuel cell, which oxidizes hydrogen to produce electricity and water.
Referring to FIG. 1, a PEM fuel cell typically includes a positive bus plate 20, an airframe 22, a cathode 23, a proton exchange membrane 26 with a catalyst layers 24 and 27 on opposing surfaces, an anode 28, a hydrogen frame 30 and a negative bus plate 32. The PEM fuel cell operates by introducing hydrogen gas at the hydrogen frame 30, the hydrogen molecules contact the catalyst 27 giving up electrons and forming hydrogen ions. The electrons travel to the cathode 23 by flowing through the anode 28, the negative bus plate 32, an external circuit 34 and the positive bus plate 20. The electrical current produced by the reaction can be used to power portable electrical devices 36 such as a laptop computers, digital cameras, personal digital assistants or hand held power tools.
The proton exchange membrane 26 allows protons to flow through, but stops electrons from passing through it. As a result, while the electrons flow through the external circuit 34, the hydrogen ions flow directly through the proton exchange membrane 26 to the cathode 23, where they combine with the oxygen molecules and the electrons to form water. The chemical equations look like the following:Anode: H2→2H++2e−Cathode: O2→2O−Overall: 2H++O−→H2O
When an H2 molecule comes in contact with the catalyst 27 preferably platinum, it splits into two H+ ions and two electrons (e−). On the cathode side of the fuel cell, oxygen gas (O2) is forced through the catalyst 24, where it forms two oxygen atoms. Each of these oxygen atoms has a strong negative charge, which attracts the two H+ ions through the PEM 26 and combines with two of the electrons from the external circuit to form a water molecule (H2O).
It should be recognized that the power demands of portable electrical devices vary over time and to operate efficiently the output of the fuel cell must be regulated to match these needs. Therefore a need exists for a method and apparatus to regulate the power produced by a fuel cell to meet the variable energy needs of portable electrical devices.